1. Field of the Invention
The present invention relates to a semiconductor device having a circuit which is configured of thin film transistors (hereinbelow, often abbreviated to “TFTs”). By way of example, it relates to the constructions of an electrooptic device which is typified by a liquid crystal display panel, and an electronic equipment in which such an electrooptic device is installed as a component.
Incidentally, here in this specification, the expression “semiconductor device” is intended to signify general devices which can function by utilizing semiconductor properties, and it shall cover all of electrooptic devices, semiconductor circuits and electronic equipment within its category.
2. Description of the Related Art
In recent years, notice has been taken of technology wherein thin film transistors (TFTs) are constructed using a semiconductor thin film (several to a few hundred [nm] thick) which is formed on a substrate having an insulating surface. The TFTs are extensively applied to ICs (integrated circuits) and electronic devices such as electrooptic devices, and it is especially hurried to develop them as the switching elements of an image display device.                In, for example, a liquid crystal display device, attempts have been made to apply TFTs to all sorts of electric circuits such as a pixel unit in which pixels arrayed in the shape of a matrix are individually controlled, a driver circuit which controls the pixel unit, and a logic circuit (including a processor circuit, a memory circuit, etc.) which processes data signals fed from outside.        Besides, there has been known a construction (system-on-panel) in which the above circuits (pixel unit, driver circuit, etc.) are packaged on a single substrate. In the pixel unit, the pixel plays the role of retaining information sent from the driver circuit. Herein, unless the OFF current of the TFT connected to the pixel is sufficiently small, the information cannot be retained, and a good display cannot be presented.        
On the other hand, in the driver circuit, a high mobility is required of the TFTs. As the mobility is higher, the structure of this circuit can be simplified more, and the display device can be operated at a higher speed.
As stated above, the TFTs arranged in the driver circuit and those arranged in the pixel unit are different in the required properties. More specifically, the TFTs arranged in the pixel unit need not have a very high mobility, but their requisites are that the OFF current is small and that the value thereof is uniform throughout the pixel unit. In contrast, the TFTs of the driver circuit located around the pixel unit take preference of the mobility over the OFF current, and their requisite is that the mobility is high.
It has been difficult, however, to manufacture the TFTs of the preferential mobility and the TFTs of the small OFF current on an identical substrate at a high productivity and without spoiling their reliabilities, by employing a fabricating method in the prior art.